1. Field of the Invention
This invention concerns synchronous digital data transmission systems and, more particularly, an echo canceller for a digital receiver for use in such transmission systems.
2. Background of the Invention
In synchronous digital transmission systems, the bit sequence resulting from the digital coding of the signal to be transmitted, is first converted into a data symbol sequence. These symbols, then, are sequentially transmitted over a transmission channel, at a so-called signalling time interval T. The transmission is carried out upon modulation of a frequency carrier f.sub.c, which modulation consists in having each symbol converted into a discrete value of one or several characteristics (amplitude, phase) of the carrier wave.
Due to the cost of the transmission channels, duplex transmissions are carried out, using two wire lines, whereas, four wires would normally be necessary, two for the emission and two for the reception. However, two to four wire conversion, and conversely, is carried out at each transmission station by means of a hybrid junction. The separation of both channels from each other could only be fulfilled with differential transformers adapted to the characteristic impedance of the transmission channel. In practice, the characteristic impedance of the transmission channel cannot be known perfectly (more particularly in the so-called switched network) and is time-varying which leads to unbalanced networks. As a consequence, undesired parasitic reflected waves, i.e. echoes, are generated within said unbalanced network. There are many types of echoes, including the so-called listener echo. This type of echo is the echo which appears in the receiver of the listening terminal. Said listener echo is derived from a transmitted signal which, after recirculating through the transmission system and therefore being delayed, adds itself to the normally received signal. This echo is a parasitic wave characterized not only by its delay but also by its phase which can be time-varying. The listener echo, therefore, is particularly difficult to control and eliminate from the received wave.